The present invention relates to a multilayer type piezoelectric filter which is suited for a wave filter circuit in a mobile radio communication device such as a portable radio communication device and an automotive telephone.
This kind of piezoelectric filter consists of a plurality of unit filter circuits, each including a series resonator and a parallel resonator which are connected to constitute an L-connection. The unit filter circuits are laminated one upon the other and disposed between an input terminal and an output terminal or a grounding terminal. The resonance frequency of the series resonators and the antiresonance frequency of the parallel resonators are adapted to correspond with each other, and the difference between the antiresonance frequency of the series resonators and the resonance frequency of the parallel resonators is held constant.
In the conventional structure of the piezoelectric filter, resonators and terminal boards are generally laminated to form a multilayer structure and disposed within a casing. This structure encounters a problem that it requires a large number of constituent parts and a difficult assembling work. On the other hand, for downsizing of a mobile radio communication device, the piezoelectric filter is required to be as thin as possible and as small in size as possible. Further, it is required that an electrical connection between each piezoelectric filter and an electrical path on a resonator mounting surface be attained with ease and the resonators be protected electrically and mechanically. To meet this requirement, it has been proposed a multilayer type piezoelectric filter wherein at least two resonators surrounded by frame-like spacers are laminated with an intermediate printed circuit board being interposed therebetween, a chamber is provided within each frame-like spacer for receiving therewithin a resonator, a top printed circuit board is disposed at the uppermost position, a bottom printed circuit board is disposed at the lowermost position, and electrode pads for connection to a printed circuit board on which the piezoelectric filter is to be mounted is formed on the lower surface of the bottom printed circuit board, whereby to form a predetermined filter circuit.
With such a structure, the piezoelectric filter is formed from plate-like elements or components which are laminated one upon the other. The piezoelectric filter can be simple in structure and compact in size. Further, the resonators are protected by the frame-like spacers and the top and bottom printed circuit boards. Thus, the piezoelectric filter has an advantage that it is strong and durable and the resonators are protected electrically and mechanically.
In the meantime, in the above described multilayer type piezoelectric filter, reflow soldering is performed when the electrode pads at the lower surface of the piezoelectric filter are electrically and mechanically connected to a predetermined electrical path on a printed circuit board on which the piezoelectric filter is to be mounted. In this instance, the lower portion of the piezoelectric filter is exposed to heat of a high temperature. Further, the frame-like spacers and the printed circuit boards are bonded together by adhesive sheets which need to be heated. Thus, the piezoelectric filter is exposed to heat of a high temperature at the time of heating of the adhesive sheets. In case the piezoelectric filter is held within an atmosphere of a high temperature, the internal pressure becomes higher since the chambers defined within the frame-like spacers and receiving therewithin the resonators are hermetically sealed. Furthermore, if there is a little water within the chambers, it is vaporized to increase the internal pressure further. By this, there is caused a problem that the top printed circuit board and the bottom printed circuit board are curved or rounded outside and therefore the outer casing constituted by those printed circuit boards is deformed or transformed.
It is accordingly an object of the present invention to provide a multilayer type piezoelectric filter which is assuredly prevented from deformation even when it is heated to a high temperature and which is strong and durable so as to maintain stable characteristics for a long period of usage.
To accomplish the above object, there is provided according to an aspect of the present invention a multilayer type piezoelectric filter comprising at least two laminated resonators, at least two frame-like spacers surrounding the resonators, respectively, at least one intermediate printed circuit board interposed between the resonators and between the frame-like spacers, a top printed circuit board disposed on the resonators and the frame-like spacers to cooperate with one of the frame-like spacers and the intermediate printed circuit board to define therebetween a first chamber within which one of the resonators is disposed, a bottom printed circuit board disposed under the resonators and the frame-like spacers to cooperate with the other of the frame-like resonators and the intermediate printed circuit board to define therebetween a second chamber within which the other of the resonators is disposed, a plurality of electrode pads formed on a lower surface of the bottom printed circuit board, a first communication hole formed in the intermediate printed circuit board for providing communication between the first chamber and the second chamber, and a second communication hole formed in the top printed circuit board for providing communication between the first chamber and the outside.
According to another aspect of the present invention, there is provided a multilayer type piezoelectric filter comprising a plurality of resonators laminated one upon the other, a plurality of frame-like spacers surrounding the resonators, respectively, a plurality of intermediate printed circuit boards interposed between adjacent two of the resonators and between adjacent two of the frame-like spacers, a top printed circuit board disposed on the resonators and the frame-like spacers to cooperate with associated one of the frame-like spacers and associated one of the intermediate printed circuit boards to define therebetween a first chamber, the intermediate printed circuit boards cooperating with the frame-like spacers to define therebetween at least one second chamber, a bottom printed circuit board disposed under the resonators and the frame-like spacers to cooperate with associated one of the frame-like spacers and associated one of the intermediate printed circuit boards to define therebetween a third chamber, the resonators being disposed within the first chamber, the second chamber and the third chamber, respectively, the frame-like spacers, the top printed circuit board and the bottom printed circuit board being bonded together and hermetically sealed at joining portions thereof, a plurality of electrode pads formed on a lower surface of the bottom printed circuit board, a plurality of first vents formed in the intermediate printed circuit boards for providing communication between the first chamber and the second chamber and between the second chamber and the third chamber, and a second vent formed in the top printed circuit board for providing communication between the first chamber and the outside.